A zoom lens used in a TV camera for broadcasting requires a high performance over the entire range of zoom and over the entire range of object distances. Furthermore, having a high zoom ratio, wide field angle, and a small size are also important features for such a zoom lens. Zoom lenses composed of four lens groups having, in order from the object side, positive, negative, negative and positive refractive power, respectively, and wherein the second lens group moves to vary the zoom ratio are disclosed in the following Japanese Laid Open Patent Applications H7-35976, H8-129134, H8-136808, and H10-161026. However, the above-mentioned prior art zoom lenses have a problem in that a ghost image is easily produced as a result of these lenses having a long exit pupil, and the ghost image that is produced adversely affects the image quality.
The ghost image appearing on conventional zoom lenses (as in the above-mentioned patent applications) is described hereinafter with reference to FIG. 8 and FIGS. 9(a)-9(c). FIG. 8 illustrates the lens element arrangement of such a conventional, four-group zoom lens at a zoom ratio of 2.times. (top), 4.times. (middle), and 6.times. (bottom). As illustrated in FIG. 8, ghost images are primarily the result of light that is reflected at the image surface then being incident on and reflected by the image side surface of the second lens group G.sub.2. This reflected light is then re-imaged by third and fourth lens groups G.sub.3 and G.sub.4 onto the image surface to form the ghost image. FIGS. 9(a)-9(c) are enlarged views of the image surface for these three zoom ratios, and illustrate the photographic image point and the ghost image at magnification power ratios of 2.times., 4.times. and 6.times., respectively.
As shown in FIGS. 9(a)-9(c), the ghost image usually appears on the opposite side of the optical axis from the actual image as a result of the image light being reflected by a convex surface of the zoom lens. Further, the size of the ghost image varies when zooming from the wide-angle end to the telephoto end. Therefore, the appearance of the ghost image is a problem in that reflected light from the actual image is again incident onto the image surface. This is especially troublesome in the case of a zoom lens as used in a TV camera where the optical system has a long exit pupil in order to accommodate optical components, such as a color separating optical system, within the space between the back surface of the zoom lens and the image plane.
Of FIGS. 9(a)-9(c), the most serious adverse affect due to ghost image light degrading the image occurs in the case illustrated in FIG. 9(b), wherein the ghost image light at the image surface is most concentrated due to the ghost image light being nearly in focus at the image surface.